Your BMI is very simple to calculate but should be used as a guideline for healthy weight only. Because muscle weighs more than fat, weight and your BMI are not the only determination of your level of health.

The BMI Formula

To calculate your BMI, simply complete the following steps. All you need is yourself, your accurate height, a scale and a calculator.

First, weigh yourself. If you normally weigh yourself once during the day at a specific time you are going to do things a little bit differently this time. You need to weigh yourself in the morning before you eat or drink (because food and liquids can affect your body weight.) Weigh yourself three times and then take an average of those weights to get an accurate figure.

Next, you need to convert your height into inches and then square that figure. For example, if you are 5'5" your height in inches would be 65 inches. Square that number (65X65) and then divide your weight in pounds by that figure. If you are 5 feet, 5 inches tall and weigh 165 pounds, your ending number would be .03905.

The next step is to take that number and multiply it by 703 (see why you need a calculator!) This step will convert the pounds and inches into the metric system. In our example, .03905 multiplied by 703 is 27.45. This figure is your BMI and it falls within one of four categories: underweight, normal range, overweight or obese. Underweight is 18.5 and below, 18.5 to 24.9 is normal, 25.0 and 29.9 is overweight and 30 and over is obese.

BMI for Children and Teens

All adults over 20 years of age can use the BMI ranges above. BMI for children and teens is calculated the same way that it is for adults, but the values are not classified the same way. Instead of charts with clearly defined values for overweight and underweight, children are compared with other children of the same age and sex.

If the BMI-for-age is less than the 5th percentile the child is underweight; If the BMI-for-age is between the 85th and 95th percentile, the child is at risk for becoming overweight; If the BMI-for-age is in the 95th percentile, the child is classified as overweight.

Here's a BMI calculator for children and teens that will determine both the BMI and the the child's BMI percentile: (http://pediatrics.about.com/cs/usefultools/l/bl_bmi_calc.htm).

Keep in mind that a BMI is only a screening tool that is used by doctors and healthcare professionals to categorize your weight. There may be other factors that contribute to your high or low number that have nothing to do with your weight, such as a long-term illness that produces an underweight BMI or lots of muscle mass that produces a high BMI.

Once you determine your BMI, you should check with your doctor if you are outside of the normal range. Your doctor can provide you with solutions to help you either gain or lose weight when needed. You should not undergo any weight loss or weight gain program without first consulting your doctor, who can offer additional testing and nutritional counseling to help you meet your goals.